The trough solar collector is a well-known collector technology used for Concentrating Solar Power (CSP) plants. As shown in FIG. 1, such a plant typically employs a large array of sun-tracking, focusing reflectors that concentrate incoming solar radiation onto a tubular conduit that contains a working fluid. The focused radiation heats the working fluid, for example an oil or other fluid. The heated fluid is piped to a central location where its accumulated thermal energy may be utilized in a conventional heat engine, for example to generate steam that drives turbines to produce electric power. In other applications, the heated fluid may be used directly, for example to where the heated fluid is used to heat water for domestic or commercial use. After its thermal energy has been utilized, the fluid may be recirculated through the collector array to be heated again.
The collector arrays may be quite large, covering several square kilometers and including thousands of collector modules, such as the module 101 shown in the simplified diagram of FIG. 1. Several modules are shown in FIG. 1, each of which has a similar construction. The field or array of collectors may be divided into parallel circuits, so that the working fluid need not be circulated through the entire collector field before it is piped to the central location, but instead may be passed through a single row of a few dozen modules during each heating cycle, for example. Many arrangements of circuits are possible. Each module typically includes a parabolic reflector 102 backed by a frame or truss system 103 on the back side of the reflector (away from the sun). The frame adds rigidity to the module. The modules are typically supported on pylons 104 that are located between the modules.
The collector modules are typically grouped into rotatable solar collector assemblies (SCAs) of several modules each, connected in a row. That is, an SCA typically includes several collector modules supported by pylons in a linear arrangement, such that each SCA can rotate about a longitudinal axis. For optimum collection efficiency, all the modules in an SCA preferably rotate in unison to track the sun during the day. Each SCA may be moved by a drive mechanism (not shown) near the center of the SCA, at an end of the SCA, or at another location within the SCA. The collector modules in an SCA are coupled to each other using a central torsion element (shaft) to couple adjacent modules. The components involved in the transfer of torque from one module to the next are sometimes referred to as a “torque transfer assembly”. FIG. 2 shows an example of the portion of a torque transfer assembly 200 of one module 101 from its back side. The shaft 201 of the torque transfer assembly 200 is typically located internal to the cross section of the frame 103, near the center of mass of the module. The shaft is typically made of a large-diameter, heavy wall pipe or tubing, and may have a machined outer surface for use with a bearing supporting the module. Attached to the shaft 201 is a heavy plate or truss structure 202 that brings forces from the large frame of one module into the relatively small cross section of the shaft 201, which transmits the torque by essentially pure torsion of the shaft 201. The torque is distributed from the shaft 201 to the frame or truss 202 of the next module via a corresponding plate of the next module (not shown in FIG. 2).
Torque from at least two different sources is transferred between modules via the torque transfer assembly 200. First, a drive mechanism located near the center of the SCA applies torque directly to those modules adjacent to the drive mechanism. For the rest of the modules in the SCA, torque is coupled from one module to the next so that the entire group of modules in the SCA rotates in unison. Second, the module arrays are also subject to wind loading, which may exert very large forces and torques on the array. Wind loading on each module is transmitted to the adjacent module. The resulting torque may be smallest at the end modules of an SCA, but may accumulate through the modules in the SCA row until the drive mechanism must resist the accumulated torsional wind loading of many modules. These torques may be as large as hundreds of thousands of Newton-meters. In order to maintain proper aiming of the array toward the sun, the drive mechanism must be able to resist and overcome the torque resulting from wind loading, and the SCA must be stiff enough that no modules deflect enough from optimum aiming that their energy collection performance is degraded significantly. While the torques are greatest near the drive mechanism, and the modules adjacent the drive mechanism must resist the largest torques, the deflection may accumulate outward from the drive mechanism, and may be greatest at the end of the SCA furthest from the drive mechanism. In order to achieve enough stiffness, the central shaft 201 must be quite large. The torque transfer assembly 200 may require a large amount of material, and may account for 20 percent or more of the structural cost of each module.
Also, the coupling of two optically-precise devices, such as the modules of an SCA, requires that the assembly be fabricated with a relatively high degree of precision for proper energy collection. An additional alignment procedure is usually required as part of the installation process to reduce rotational misalignment between modules.
Another significant issue in the design of an SCA is the accommodation of thermal expansion and contraction. The SCAs are often quite large and may be deployed in environments with significant temperature extremes. For example, an SCA may be 150 meters long, and its length may change as much as 20 centimeters between the temperature extremes that it experiences. Since the SCA is typically anchored in the middle at the drive mechanism, which is not free to move, each end may move as much as 10 centimeters during a temperature cycle. Because of the high shear and torque loads on the torque transfer shafts 201, it is typically cost-prohibitive to employ means such as splined or telescoping shafts to absorb the longitudinal thermal expansion and contraction. Typically, thermal expansion and contraction are accommodated by simply allowing the entire row to expand and contract from the central anchor. This requires over-sizing the length of the torque transfer shafts to provide clearance between each module frame and its supporting structure at both temperature extremes. This further increases the cost of the material used for the shaft, and the increased shaft length further reduces the shaft stiffness, exacerbating the deflection issues described above.